For many years, it has been conventional practice to interconnect telephone instruments to central switching devices such as private automatic branch exchanges, key systems and the like by inexpensive twisted pair transmission lines. These transmission lines consist of two elongated insulated conductors which are twisted together along their lengths. The instruments and switching devices, until very recently, utilized low frequency analog signals typically in a frequency range of 0-4 KHZ. The twisted pair transmission lines were not only inexpensive and technically acceptable for transmitting such signals but they could also be installed and removed or replaced easily and inexpensively.
New telephone instruments and other types of equipment incorporating telephone functions, now referred to as terminals, as well as new central switching devices utilize digital rather than analog signals. However, the signal transmission parameters of twisted pair transmission lines are inadequate for digital signal transmission. If twisted pair transmission lines are used to transmit digital signals, excessive signal degradation results. Such signal degradation can be corrected by connecting expensive equipment to these lines, but twisted pair transmission lines are still unsuitable for general use in transmitting digital signals because of the high cost of such equipment.
When digital equipment is to be installed with new transmission facilities it is customary to use coaxial cables, rather than twisted pair transmission lines since coaxial cables have signal transmission parameters which are suited to transmitting digital signals. However, when twisted pair transmission lines have already been installed, expensive and time consuming replacement procedures are required to remove these lines and replace them with coaxial cables.
The present invention overcomes the requirement of such replacement procedure by providing new and inexpensive equipment which cooperates with a twisted pair transmission line in such manner that it can be used to transmit digital signals over extended distances, e.g. several miles, without causing appreciable signal degradation.